What does the notation $[m]=\{1, \ldots, m\}$ mean in the equation of the empirical error?
This is a commonly used notation in theoretical computer science.

$[m]$ is not the variable $m$, but is instead the set of integers from $1$ to $m$ inclusive. The empirical error equation thus reads in English:

The cardinality of a set consisting of the elements $i$ of the set of integers $[m]$ such that the hypothesis given input $x_i$ disagrees with label $y_i$, normalized by $m$.